High resolution three-dimensional (3D) optical imaging instruments, such as confocal microscopes and optical coherence tomography systems, are important tools in biological and medical research. During the last decade, volume holographic imaging systems (VHISs) have been developed which use the wavefront selection properties of a volume hologram to select multiple images from respective multiple object depths. See W. Liu et al. “Real-time spectral imaging in three spatial dimensions,” Opt. Lett. 27, 854-56 (2002); A. Sinha et al., “Volume holographic imaging in the transmission geometry,” Appl. Opt. 43, 1533-51 (2004) (herein “Sinha I”); Z. Li et al., “Volume holographic spectral imaging,” Proc. SPIE 5694, 33-40 (2005); A. Sinha et al., “Broadband volume holographic imaging,” Appl. Opt. 43, 5215-5221 (2004) (herein “Sinha II”); Y. Luo, “Optimization of multiplexed holographic gratings in PQ-PMMA for spectral-spatial filters,” Opt. Lett. 33, 566-68 (2008) (herein “Luo I”); P. J. Gelsinger-Austin et al., “Optical design for a spatial-spectral volume holographic imaging system,” Opt. Eng. 49, 043001 (2010); Y. Luo, “Simulation and experiments of aperiodic and multiplexed gratings in volume holographic imaging systems,” Opt. Express 18, 19273-19285 (2010) (herein “Luo II”); W. Sun et al., “Rainbow volume holographic imaging,” Opt. Lett. 30, 977-978 (2005); and, Psaltis et al., U.S. Pat. No. 7,158,228, all of which are hereby incorporated by reference in their entirety. Such VHISs have achieved lateral and depth resolution of ˜3 μm and ˜12 μm, respectively, when using monochromatic illumination and standard lens components. Liu et al., Sinha I, Sinha II, Luo I, Gelsinger-Austin et al., and Lou II, id. However, a disadvantage of using a monochromatic source is that lateral scanning is required in order to capture the complete image field.
Using a broadband source has been proposed and implemented with the goal of avoiding mechanical scanning while maintaining the resolution achieved using mono-chromatic sources. However, in practice it has been observed that the utilization of a broadband illuminator dramatically reduces the depth sectioning capabilities of VHIS. Sinha II, Luo I, Gelsinger-Austin, Luo II and Sun et al., Id.
An approach to improve depth resolution that utilizes a rainbow illumination pattern produced by the diffraction of a broadband source on an external grating has been proposed. Sun et al., id., and Sun et al. U.S. Pat. No. 7,262,889 (the Sun '889 Patent) which is hereby incorporated by reference in its entirety. This type of VHIS configuration has been shown capable of improving the depth resolution to values >200 μm. There are limitations for improving depth resolution beyond that value using this configuration, because it requires accurately matching the wavefront of the external diffraction gratings (illumination hologram) and internal diffraction gratings (imaging hologram) and because there must inevitably be a finite angle between the optical axis of the illumination hologram and the optical axis of the imaging hologram.
The basic configuration of a VHIS 10 is illustrated in FIG. 1. The system (having coordinates x, y, z) consists of an objective lens 12, a volume hologram 14 placed in the Fourier plane of the objective lens 12, and a collection lens 16. The objective lens 12 receives light from an object 18 disposed in object space (denoted by coordinates x′, y′, z′), and the collection lens 16 forms a real image 20 of the object in image space (denoted by coordinates x″, y″, z″). The hologram 14 comprises a thick hologram having angle-multiplexed planar and spherical wave gratings having high angular and spectral selectivity. A broadband source is used to illuminate the object. A system of this type is disclosed in the Sun '889 Patent.
Because of the high selectivity, each grating, if illuminated with a monochromatic point source, would select a specific wavefront that originates at a corresponding depth from within object space. Multiplexing several gratings into the same volume allows mapping points from multiple depths in object space to distinct, corresponding locations on the image plane (x″, y″) in image space as shown by respective marginal rays 17 and 19. For each plane in object space the mapping is determined by two properties of the volume hologram 14: (1) its spatial degeneracy; and (2) its angular dispersion. Because of the first property, the wavefront of a point source at any position along the y′ axis satisfies the Bragg phase-matching condition of the hologram 14 and therefore is diffracted to the collection lens 16. This diffraction is responsible for the y axis field of view (FOV) of the system
The FOV along the spatial degeneracy axis y axis does not follow a straight line along the y′ axis, as represented by the cylinder axis shown in FIG. 1. Rather, it follows hyperbolic curves such as δλ0, δλ1, δλ2, δλ3, δλ4 and so forth, as illustrated in FIG. 3 and described in Castro et al., “Analysis of diffracted image patterns from volume holographic imaging systems and applications to image processing,” Appl. Opt. 50, 170-176 (2011), hereby incorporated by reference in its entirety. The lateral resolution in the y axis depends mainly on the numerical aperture (NA) of the objective lens 12.
The angular dispersion of the hologram 14 and the spectral bandwidth of the readout source in image space, such as a CCD array, determine the FOV in the dispersive axis, that is, x axis. The lateral resolution along the x axis depends on the spectral selectivity of the hologram, which can be improved by optimizing its fabrication parameters. Luo I, Gelsinger-Austin et al., and Luo II, id., and Castro et al., “Resolution dependence on index modulation profile and effective thickness in volume holographic imaging systems,” Appl. Opt. (1 Mar. 2011, Vol. 50, No. 7, pp. 1038-46), hereby incorporated by reference in its entirety. For a VHIS operating with monochromatic illumination, the depth selectivity depends on the NA of objective lens 12 and the angular bandwidth of the hologram 14.
VHIS prototypes using monochromatic sources have achieved lateral resolution of 2:5 μm and depth resolution of ˜12 μm. Liu, id. However, a drawback is that in this configuration, scanning is required to capture the x axis FOV. However, when a broadband light source is utilized as an illuminator in the VHIS, the depth selectivity is essentially lost.
A VHIS 100 using multi-spectral, or “rainbow,” illumination to improve the selectivity without requiring lateral scanning is illustrated in FIG. 2. This approach requires two sets of gratings and lenses: one to provide the rainbow illumination and the other for imaging. Thus, a beam of multi-spectral illumination 102 is applied to illumination hologram 104, which diffracts the multi-spectral light by different angles depending on the wavelength of the light, as shown at 106 by marginal rays 107 and 109. An illumination lens 108 then focuses light of different wavelengths to different locations in its image space. The image space of the illumination lens 108 is the object space 110 of an objective lens 112 that collects light from points in that object space, collimates it and illuminates an imaging hologram 114. The hologram 114 discrimination selects light from only one point in object space for a given wavelength as shown by marginal rays 111 and 113 and directs light 120 through a collection lens (not shown) to a corresponding unique point on a plane in image space.
The rainbow illumination is produced by the dispersive properties of the illumination hologram 104. Ideally, the illuminated plane 116 should overlap the object plane 118 along the complete FOV of both the illumination lens 108 and the objective lens 112. Also, the spectral dispersion produced by each set of optical elements should match. This ideal condition cannot be fully attained with the layout shown in FIG. 2. The challenge of dispersion-matching between gratings in the illumination hologram 104 and the gratings in the imaging hologram 114 limit the depth resolution not only in the optical axis but also over the complete FOV in the object plane. Even if that challenging condition could be satisfied, an overlap between the illuminated plane 116 and the object plane 118 is required. For imaging systems using an objective lens with NA>0:5, this latter condition is not attainable with external illumination and a thin grating with poor selectivity properties must be used. For example, FIG. 2 shows that there is a tilt angle, ε, between the object plane and the illuminated plane. This angle, which takes values of ˜60° for lenses with NA ¼ 0:5, reduces significantly the region in which the object and the illuminated plane overlap.
In view of the foregoing, there has been an unmet need for a VHIS which provides for depth sectioning of an object, eliminates the need for a mechanical scanning apparatus to cover the FOV of the system, provides high lateral and depth resolution, and provides for a high image contrast ratio.